It has long been known that various polymers tend to collect electrostatic charges on their surface. This tendency creates difficulty in the handling of the polymer and of articles made therefrom, for it occurs during storage, as well as in the course of processing the polymers into shaped forms, such as filaments, sheets, films, and molded plastics.
Such electrostatic charges cause dust and dirt particles to adhere to the plastic surfaces and, also, the plastic surfaces to adhere to each other or to the equipment used in processing. Under certain circumstances, the accumulated charges may give rise to sparks, with an attendant fire hazard. The tendency toward the building of electrostatic charges is especially marked in the case of polymers and copolymers made from ethylenically unsaturated monomers, such as polymers and copolymers of vinyl chloride, vinylidene chloride, styrene, and the various polyolefins, such as polyethylene, polypropylene, and polybutylene.
Among the approaches taken in the prior art to reduce the tendency of plastic materials toward electrostatic charging has been either to coat the plastic material with an antistatic composition, or to incorporate it into the body of the plastic material. The latter expedient is generally considered to be more effective. Compounds which have been proposed for this purpose include polyalkylene glycols and their esters and ethers, and a wide variety of amines and amides.
Many of the amine or amide antistatic agents are liquids which, when incorporated into the resin compositions, migrate to the surface at an undesirably rapid rate, causing losses by evaporation, diminished antistatic effectiveness, development of undesirable odors, and adversely affecting the surface properties of the plastic, for example, by promoting cracking or crazing.
Attempts to use solid amine antistatic agents with polymers were unsuccessful in that with time the ethoxylated amine particulates would agglomerate and present feeding problems.
Another approach to avoid the use of liquid antistatic agents is disclosed in U.S. Pat. Nos. 4,210,556, granted July 1, 1980, and 4,314,040, granted Feb. 2, 1982, to Castro et al, wherein it is disclosed to incorporate the liquid amine in a base resin such as polyproylene particulates of 1/8 to 1/16 inch in diameter. This approach involves a costly process and the effective amount of amine antistatic agent is generally no more than about 50% activity in commercial application.
In the plastic extrusion field, it is known that micron sized silica gels are useful as anticaking agents in plastic extrusion operations.
It is therefore, a principal object of the present invention to provide a novel solid antistatic composition and a method for making the composition.
It is another object of the present invention to provide a composition as aforesaid in which there is a highly effective amount of antistatic agent per unit weight of composition.
It is another object of the present invention to provide a composition as aforesaid which is readily and inexpensively processed from relatively inexpensive materials.
It is another object of the present invention to provide an antistatic agent which remains free flowing over extended periods of time.
It is still a further object of the present invention to provide a solid antistatic agent which is useful in and acceptable with food grade polyolefin packaging materials.
The aforesaid, as well as other objects and advantages, will become apparent from a reading of the following description and the adjoined claims.